I think it's really smart to ask a prospective dock builder how deep they plan to drive the poles. Or, at least, given the terrain / rock in your particular lake bed, how deep they anticipate having to sink the poles.

From my personal experience of looking at tons of homes around the lake, I can tell you that more than a few don't drill down far enough and just plop the pilings in mud. The dock starts to fail relatively quickly and it's obvious they didn't go down far enough. First you see it on the poles that are supposed to hold the floating pier, then it spreads to the fixed. That's what seperates a weekend carpenter from someone with marine construction experience.